The present invention relates generally to a cathode-ray tube apparatus, and more particularly to a color cathode-ray tube apparatus equipped with a velocity modulation coil.
There is known a practically used color cathode-ray tube apparatus equipped with a velocity modulation coil for clarifying the contour of an image. The velocity modulation coil is mounted on an outer surface of a neck located behind a deflection yoke, thereby to enhance the sharpness of an image.
The velocity modulation coil may be disposed at any position where electron beams will pass, if the whole system is considered. It is necessary, however, to dispose it where no interference of generated magnetic fields will occur between itself and the deflection yoke. Accordingly, there is no choice but to dispose the velocity modulation coil at a predetermined position on the cathode side of an anode electrode.
Taking the above into account, the velocity modulation coil is normally disposed around a location where a focus electrode is provided. In this case, however, the frequency of current flowing in the velocity modulation coil is high, and a magnetic field generated from the velocity modulation coil causes an eddy current in the focus electrode. Since the eddy current suppresses generation of a magnetic flux of the velocity modulation coil, which acts in the focus electrode, the velocity modulation effect is disadvantageously reduced.
In order to intensify the magnetic field of the velocity modulation coil, two methods are available: to increase the current flowing in the velocity modulation coil, or to increase the number of turns of the velocity modulation coil. In the case of the former, the diameter of wire of the coil needs to be increased, and a greater power consumption is required to supply a greater current. As a result, a load on the circuit, as well as the cost, will increase. In the case of the latter, the thickness of the velocity modulation coil increases, and the adjustment performance for a purity convergence magnet deteriorates. Although the magnetic field can theoretically be intensified by adjusting the position of the velocity modulation coil, the position of the coil cannot freely be changed because of positioning restrictions on actual design, as mentioned above. Besides, in general terms, if a magnetic field for correcting the contour of an image is intensified by some method, the action of the magnetic field on electron beams increases and the amount of a leak magnetic field also increases. Consequently, a problem of an electromagnetic wave fault may arise.
On the other hand, there are known electron gun structures, as disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 62-21216, etc., which embody a method of causing a magnetic field of the velocity modulation coil to effectively act on electron beams, without intensifying this magnetic field. In these structures, an electrode in a region where the velocity modulation coil is positioned, which is normally a single electrode or an integral electrode of tightly welded plural electrode components, is divided into electrode members with spaces provided thereamong, and these electrode members are electrically connected by means of lead wire.
The spaces among the divided electrode members of the electrode function to suppress an eddy current caused in the electrode by the magnetic field of the velocity modulation coil, and to let the magnetic field of the velocity modulation coil permeate into the electrode and act on the electron beams, thus enhancing the velocity modulation effect. In this method, however, a welding work for lead wire is necessary in order to electrically connect the electrode members of the electrode. There is a possibility of a problem of work efficiency and deformation of the electrode members at the time of welding lead wire. Moreover, since the electrode members are spaced apart, the strength of holding of the electrode members may become deficient, the electrode members may be displaced relative to the axial direction, or the electric field from the inner wall of the neck may permeate.
Jpn. Pat. Appln. KOKAI Publication No. 10-172464, etc. disclose electron gun structures as other countermeasures. In the methods according to these countermeasures, slits are formed in an electrode in a region where the velocity modulation coil is positioned. The slits function to suppress an eddy current caused in the electrode, and to let the magnetic field of the velocity modulation coil permeate into the electrode via the slits and act on the electron beams, thus enhancing the velocity modulation effect. In these methods, however, the formation of the slits may decrease the strength of the electrode, degrade the precision in dimension of the electrode, e.g. circularity of electron beam passage holes, and give rise to deformation of the electrode at the time of assembly.
As has been mentioned above, in order to obtain an image with high sharpness, it is necessary to cause the magnetic field of the velocity modulation coil to effectively act on the electron beams. However, this magnetic field causes an eddy current in the electrode of the electron gun assembly, and the eddy current suppresses the magnetic field of the velocity modulation coil and degrades the velocity modulation effect.
In order to solve these problems, there are the prior-art methods wherein an electrode in a region where the velocity modulation coil is positioned, which is normally a single electrode or an integral electrode of tightly welded plural electrode components, is divided into electrode members with spaces provided thereamong, and these electrode members are electrically connected by means of lead wire, or wherein slits are formed in an electrode in a region where the velocity modulation coil is positioned. These methods, however, have the problems in that the precision in dimension of the electrode deteriorates or the electrode may deform due to the decrease in strength of the electrode, and the electric field of the neck may permeate.